This invention relates to methods and systems for processing videotape, and more particularly to correcting a discontinuous 2-3 field pattern that resides on the videotape.
In general, telecine machines that operate at 60 field/sec (actually, 59.94 fields/sec) employ a 3:2 pulldown convention to convert film media, which runs at 24 frame/sec (actually, 23.976/sec), to video media, which runs at 60 television field/sec (actually, 59.94 fields/sec). Specifically, a two-field video sequence and a three-field video sequence are alternately generated, with each field sequence corresponding to a film frame. These video fields are interlaced in that the film frames are scanned, such that alternating odd and even fields are generated, with the lines of the odd fields interleaved with the lines of the even fields. For example, a film frame can be scanned to generate a two-field video sequence characterized by an even field and then an odd field (even/odd). The next film frame can be scanned to generate a three-field video sequence characterized by an even field, then an odd field, and then an even field (even/odd/even). The respective first and second even fields in this three-field video sequence are duplicates. The next film frame can be scanned to generate a two-field video sequence characterized by an odd field and then an even field (odd/even). The next film frame can be scanned to generate a three-field video sequence characterized by an odd field, then an even field, and then an odd field (odd/even/odd). The respective first and second odd fields in this three-field video sequence are duplicates. This pattern then repeats for the next four film frames and so on.
The two/three-field video sequence is sometimes disrupted, such as, e.g., when the video is edited without regard to the video sequence. These disruptions in the video sequence can cause difficulties during processing that requires manipulation of the two/three-field sequence. For example, it is sometimes desired to convert 525 line, 60 fields/sec video to 625 line, 48 fields/sec video. During this procedure, the two/three-field video sequence is converted to a repeating two-field video sequence by removing the duplicate field from each of the three-field video sequences, and, if needed, swapping the order of the two-field video sequence, thereby generating a repeating two-field video sequence characterized by an even field and then an odd field through the entirety of the video (even/odd), or alternatively, an odd field and then an even field through the entirety of the video (odd/even). The 525 line resolution of the 525 line, 60 field/sec video is then interpolated to produce the 625 line, 48 field/sec video with 625 lines of resolution. The resulting 625 line, 48 field/sec video is recorded at 24 frames/sec, which is then played at 25 frames/sec, which is the normal 625 line, 50 field/sec video when viewed. When there is a disruption in the two/three-field video sequence nor to conversion, the two-field sequence subsequent to conversion will sometimes change dominance. That is, the repeating odd/even video sequence changes to a repeating even/odd video sequence. Because a television system cannot process an odd/odd video sequence (or an even/even video sequence), an even field (or odd field) by itself, or an even/odd/even video sequence (or an odd/even/odd video sequence) must be located at the change in dominance. As a result, the video is degraded.
Another example of a process that requires the manipulation of the two/three-field video sequence is the conversion of video to a digital video disk (DVD). To save memory, the two/three-field video sequence is converted to a repeating two-field video sequence in much the same manner described above. The two-field video sequence is then compressed into a motion pictures expert group (MPEG2) format. The DVD player then restores the two/three-field video sequence during playback on the television system. Again, however, during conversion, the field dominance of the two-field video sequence may change, thereby degrading the DVD.
Thus, it would be desirable to provide methods and systems to correct a disrupted two/three-field video sequence.
This present invention comprises novel methods and systems for correcting a discontinuous 2-3 field sequence within a disrupted video signal. A 2-3 pattern fixer constructed in accordance with the present invention can be operated in a one-pass mode and/or a two-pass mode. In a one-pass mode, the disrupted video signal is analyzed to generate correction information, which is used to correct the disrupted video signal as it passes through the 2-3 pattern fixer, preferably in real time, resulting in an undisrupted video signal with a continuous 2-3 field sequence. In a two-pass mode, the disrupted video signal is analyzed to generate correction information, which is then stored. This correction information is then used to correct a duplicate of the disrupted video signal, resulting in an undisrupted video signal with a continuous 2-3 field sequence.
In a preferred embodiment of the present invention, a 2-3 field pattern fixer includes a field sequence detector, a field sequence analyzer, a field sequence generator and a multiple delay tap circuit. The field sequence detector receives the disrupted video signal and generates a series of field difference values in response thereto by sequentially comparing each of the fields with a field two fields previous. The field sequence analyzer analyzes the series of field difference values and generates field sequence to determine one or more discontinuities within the 2-3 field sequence of the disrupted video signal. The field sequence generator uses this information to generate field sequence reorganization information in the form of a correction signal. In the preferred embodiment, the correction signal comprises a sequence of delays. The correction signal is preferably generated, such that a cumulative delay within the undisrupted video signal is minimized and the number of odd field delays are minimized. The multiple delay tap circuit then applies the correction signal to a video signal to generate an undisrupted video signal having a continuous 2-3 field sequence. That is, selected fields of the disrupted video signal or duplicate of the disrupted video signal are delayed in accordance with the correction signal, thereby resulting in the undisrupted video signal. The multiple delay tap circuit can optionally include at least one cross-fader to cross-fade between an odd-delayed field and an even-delayed field, thereby minimizing any blur caused by odd field delays.
This particular embodiment of the 2-3 field pattern fixer can be operated in the one-pass mode. In this connection, the disrupted video signal is analyzed to determine one or more discontinuities with the discontinuous 2-3 field sequence. The discontinuities can be determined by detecting a scene change and a phase change within the 2-3 field sequence. Field sequence reorganization information, and in particular, correction signals are then generated based on these discontinuities. These correction signals are then applied to the disrupted video signal as it passes through the 2-3 field pattern fixer, thereby generating an undisrupted video signal having a continuous 2-3 field sequence from the disrupted video signal.
The 2-3 field pattern fixer can optionally include a first-in-first-out (FIFO) memory and a time code comparator, allowing the 2-3 field pattern fixer to operate in a two-pass mode. The FIFO is coupled between the field sequence generator and the multiple tap delay circuit and can store several correction signals. The time code comparator is operatively coupled to the FIFO and the multiple delay tap circuit to coordinate the timing of the correction signals as they are input into the multiple delay tap circuit. The time code comparator receives at a first input a current time code of the duplicated disrupted video signal, and at a second input, a trigger time code generated in the field sequence generator and stored in the FIFO. The trigger time code corresponds with the time code during which the next correction signal in the FIFO will be initially applied to the duplicated disrupted video signal. In response thereto, the time code comparator generates a trigger signal that is input into the FIFO and the multiple delay tap circuit.
In the two-pass mode, the disrupted video signal is analyzed during the first pass to determine one or more discontinuities with the discontinuous 2-3 field sequence. Field sequence reorganization information, and in particular, correction signals and corresponding trigger time codes are then generated based on the discontinuities and associated time codes. These correction signals and corresponding trigger time codes are stored in the FIFO. During the second pass, the duplicated disrupted video signal is received by the multiple tap delay circuit. When the current time code of the duplicated disrupted video signal and the first trigger time code match, the time code comparator sends a trigger signal to the multiple delay tap circuit to begin applying the correction signal to the duplicated disrupted video signal. The trigger signal is also sent to the FIFO to advance the next correction signal for subsequent use by the multiple delay tap circuit, and the next trigger time code to the time code comparator. When the current time code of the duplicated disrupted video signal matches the next trigger time code, the time comparator again sends a trigger signal to the multiple delay tap circuit to begin applying the next correction signal. This process is then repeated until the last correction signal in the FIFO has been applied to the duplicate disrupted video signal. Each of the correction signals are preferably applied to the duplicated disrupted video signal at a field rate equal to the field rate of the disrupted video signal.
Other and further objects, features, aspects, and advantages of the present invention will become better understood with the following detailed description of the accompanying drawings.